A crankcase lubricant is an oil used for general lubrication in an internal combustion engine where an oil sump is situated generally below the crankshaft of the engine and to which circulated oil returns. It is well known to include additives in crankcase lubricants for several purposes.
Phosphorus in the form of dihydrocarbyl dithiophosphate metal salts have been used as extreme pressure, antiwear and antioxidant additives in lubricating oil compositions for internal combustion engines. The metal may be an alkali or alkaline earth metal, or aluminum, lead, tin, molybdenum, manganese, nickel or copper. Of these, zinc salts of dihydrocarbyl dithiophosphate (ZDDPs) are most commonly used. While such compounds are particularly effective antioxidants and antiwear agents such compounds introduce phosphorus, sulfur and sulfated ash into the engine which not only contaminates and shortens the service life of exhaust gas after-treatment devices but also creates environmental concerns. Such exhaust gas after-treatment devices may include catalytic converters, which can contain one or more oxidation catalysts, NOX storage catalysts, and/or NH3 reduction catalysts; and or a particulate trap.
Oxidation catalysts can become poisoned and rendered less effective by exposure to certain elements present in engine exhaust gases, particularly by exposure to phosphorus and phosphorus containing compounds introduced into the exhaust gas by the degradation of phosphorus containing lubricant additives. Reduction catalysts are sensitive to sulfur and sulfur containing compounds in the engine exhaust gas introduced by the degradation of both the base oil used to blend the lubricant, and sulfur containing lubricating additives. Particulate traps can become blocked by metallic ash, which is a product of degraded metal-containing lubricating oil additives.
In response to these problems, OEM specifications for “new service fill” and “first fill” lubricants have continually sought to reduce the maximum allowable limits of phosphorus, sulfur and sulfated ash (SAPS) contents of lubricating oil compositions. At the same time, such OEM specifications also stipulate that the lubricating oil composition must provide adequate lubricating performance. With the first licensed use of ILSAC (International Lubricant Standardization Approval Committee) GF-1 in October 1992, phosphorus levels were limited to no more than 1200 parts per million (ppm), with GF-3 in July 2001 to 1000 ppm and with GF-4 in January 2004 to 800 ppm. However, even at these reduced levels of phosphorus, contamination of exhaust gas after-treatment devices, especially oxidation catalysts, is still an issue.
Suitably, lubricating oil compositions that exert a minimum negative impact on exhaust gas after-treatment devices must be identified.